The present invention generally relates to drilling operations conducted by the petroleum industry for the recovery of oil and/or gas from subterranean formations. More particularly, the invention relates to a method of minimizing fluid loss from drilling fluids which are circulated through the wellbores penetrating such formations.
In conventional well drilling operations for the recovery of oil and/or gas from permeable subterranean formations, it is the usual practice to circulate a drilling fluid downwardly through the drill pipe, outwardly through the bit mounted at the end of the pipe and upwardly through the annulus thereof to the surface for the purpose of cooling the drill pipe and bit and transporting cuttings out of the bore hole. Water and various brines are among the commonly used drilling fluids for this purpose.
In most drilling operations, circulation of the drilling fluid is accomplished by a loss of fluid from the wellbore into the porous or fractured formation. "Fluid loss" is thus generally defined as the migration of the liquid portion of a drilling fluid or mud into the surrounding formation. Such fluid loss is undesirable because it tends to weaken the stability of the formation.
Many additives for drilling fluids and muds are known in the art for purposes of fluid loss control. U.S. Pat. No. 3,993,570 discloses the use of starch and organic starch derivatives to impart fluid loss control to wellbore fluids. Carboxymethyl cellulose and various clays are also known for this purpose. Bridging particles comprised of solid carbonate powders which serve to "bridge" across or plug the pores of a permeable formation are disclosed in U.S. Pat. No. 3,689,410. The bridging paticles are incorporated into the circulating drilling fluid and deposited therefrom onto the porous formation. U.S. Pat. Nos. 3,878,141 and 3,785,438 to Jackson et al. disclose hydroxyethyl cellulose as a fluid loss additive when used in conjunction with bridging particles composed of solid thermoplastic resins. The molar substitution of such hydroxyethyl cellulose material is not disclosed in the patents; high M.S. material being presumably as effective as low M.S. material. Moreover, to the extent that claim 1 of each of the aforementioned Jackson et al. patents states that hydroxyethyl cellulose serves as a fluid loss additive, such teaching is apparently negated by the Abstract of each of the patents which in referring to the invention therein disclosed states that "fluid loss control is established by the use of colloidal additives such as lignosulfonates . . . " (see lines 1-3 of the second paragraph of the Abstract), the inclusion of hydroxyethyl cellulose in the fluid of the invention being presumably for its known function as a solution thickener.
"Cellosize" hydroxyethyl cellulose manufactured by Union Carbide Corporation is a known viscosifier for oil and gas well drilling, completion and workover fluids. The molar substitution ("M.S.") of the commercially available material is 2.0; "molar substitution" as used herein being defined as the average number of ethylene oxide molecules that have reacted with each anhydroglucose unit of the cellulose molecule. This definition of "M.S." is in accord with that set forth, for example, in U.S. Pat. No. 3,284,353. Other commercially available hydroxyethyl cellulose materials which have heretofore been suggested for use in oil well drilling applications have an M.S. of 2.0 or higher. See, for example, U.S. Pat. No. 3,284,353 which discloses hydroxyethyl cellulose having an M.S. of 2.5 as a drilling mud additive.
The prevalent use of hydroxyethyl cellulose (HEC) material as a viscosifier for drilling fluids is attributable to its enhanced thickening properties and solubility characteristics at M.S. values of 2.0 or above. Since it is generally believed in the art that fluid loss is reduced at increased fluid viscosities (see U.S. Pat. No. 3,853,903 at column 2, lines 66-68) relatively high viscosity fluids are generally desired provided such viscosity is consistent with other desired flow characteristics. Hence, there has been no incentive in the prior art to use HEC materials having M.S. values substantially below 2.0 (corresponding to lower solubility of the HEC additive and less viscous drilling fluids) for oil well applications.
While HEC has gained commercial acceptance as a viscosifier in a variety of drilling fluids, it has heretofore been unable to minimize fluid loss in such fluids to an acceptable level. This is evidenced by the fact that when drilling fluids containing commercial hydroxyethyl cellulose in normal oil well concentrations are tested for fluid loss using the standard API fluid loss test (hereinafter described), such fluids exhibit inordinately high rates of fluid loss. Specifically, the aforementioned tests generally result in a "blowout", commonly defined as an API fluid loss in excess of 350 ml/30 min.; fluid loss rates above 100 ml/30 min. being generally considered commercially unacceptable.
As used throughout the specification and claims, the fluid loss of a drilling fluid is defined in terms of the API fluid loss test commonly used in the petroleum industry which is described in "Standard Procedure For Testing Drilling Fluids", Section 3, Filtration, American Petroleum Institute, API RP 13B, 7th Edition, April 1978, pages 8-9. The results of such tests are expressed in ml of filtrate produced in 30 minutes.